Add code to give better error message when using an undefined RDom dimension.
(Fixes https://github.com/halide/Halide/issues/332 )

Add test for above.

Remove use of double underscores in identifer as these are reserved in C++.
Fix spelling error, wording, in comment.
Merge pull request #333 from halide/undefined_rvar_error_message

Add code to give better error message when using an undefined RDom dimen...